This invention relates to a heat-sensitive transfer sheet which exhibits excellent thermal transferability and which can provide sharp printing on paper to be transfer printed.
In recent years, heat-sensitive transfer recording processes wherein a thermal head is used have been widely used because such processes are advantageous in that they produce no noise, the apparatus therefor is relatively inexpensive and compact, and maintenance is easy. As a heat-sensitive transfer sheet used in such heat-sensitive recording processes, there have been proposed heat-sensitive transfer sheets each having a transfer layer which is obtained by applying a composition onto one surface of a substrate having excellent thermal conductivity and heat resistance such as capacitor paper, polyester films and the like wherein the composition is obtained by melting and incorporating a coloring agent into waxes such as natural wax, synthetic wax and the like as a binder.
Generally speaking, performance characteristics required for the heat-sensitive recording binder include the property of preventing dark reaction prior to use; resistances such as wear resistance, deflection resistance, heat resistance, cold resistance, and chemical resistance; melting characteristic during heat printing; prevention of unnecessary color development through heat transmission to the non-image area; and the like. Binders which meet these requirements to some extent and which are being currently used include waxes such as paraffin wax, microcrystalline wax, polyethylene wax, beeswax, white Japan wax, carnauba wax, montan wax, ceresin wax, castor wax, and the like; stearic acid and derivatives thereof such as stearic acid amide, metal stearate and the like; and higher fatty acid amides and the like.
However, even if these waxes which have been predominatly used in the transfer layer in the prior art as the binder are a single material, practically, such waxes comprise a mixture of compounds having different molecular weights. When such a wax is heated, the melting point range from an initial point wherein the wax starts to melt to an end point wherein the wax is completely molten is broad, i.e., from 5.degree. to 10.degree. C. Accordingly, the temperature of the thermal head may give rise to insufficient transfer, or it may be impossible to avoid bleeding of the printing resulting from the melting of the compounds having relatively low melting point which is present in the transfer layer near the printed portions. Therefore it is difficult to transfer a sharply outlined character or figure.
When natural wax is used, the composition may vary, depending upon the place of production and weather conditions, and therefore it is difficult to produce products of constant quality. Furthermore, damage easily occurs because the wax is soft at room temperature.